Size-extensivity holds trivially for exact wave functions. For approximate wave functions, however, size-extensivity is not always observed. We now examine size-extensivity for the linear variational model of Section 4.2.3. We shall find that, for this simple model, size-extensivity may be imposed by a careful construction of the variational space for the compound wave function. For ease of presentation, we assume that aU wave functions are real. [Pg.129]

Thus, for a manifestly separable compound wave function, the interaction parameters Pab become 0 and the wave function turns into a product wave function as the interactions between A and B vanish. Manifestly separable wave functions do not require the use of interaction parameters Pab to describe noninteracting systems size-extensively and the number of variational parameters therefore scales linearly with the number of noninteracting. systems. Obviously, the exponential form of the wave function is manifestly separable, whereas the linear expansion suffers from the lack of this property. [Pg.134]

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